The internal intercostals run at right angles to the external intercostals. Their oblique direction depresses the ribs during forced expiration. The parasternal portion (interchondral) may assist inspiration.
| Origin | Upper border of each rib — inner surface |
|---|---|
| Insertion | Lower border of the rib above — fibres run obliquely backward and downward (opposite to external intercostals) |
| Nerve Supply | Intercostal nerves (T1-T11) |
| Blood Supply | Posterior and anterior intercostal arteries |
| Actions | Depress the ribs during forced expiration; Stabilise the intercostal space against outward bulging during expiration |
|---|
Internal intercostals are the layer protecting the neurovascular bundle — the intercostal nerve, artery and vein run between the internal and innermost intercostals. Thoracentesis and chest drain insertion must pass deep to the internal intercostals to enter the pleural space, ideally just above the upper rib border to avoid the neurovascular bundle.
The intercostal space is palpable between ribs — the internal intercostals form the deeper layer.
Safe thoracentesis requires needle insertion just above the rib lower border to avoid the intercostal neurovascular bundle running between the internal and innermost intercostals.
